The present invention relates to a lip seal for sealing a shaft in various rotary machines, more particularly, a technology for improving a pressure resistance under a condition that a pressure of sealing object fluid becomes high prominently and a sealing capability when the shaft stops.
Conventionally, as an apparatus for sealing a shaft, there was a lip seal 100 as shown in FIG. 5. The lip seal 100 has a structure that an elastomer lip member 102, a pressure resisting metal back-up ring 103 which supports the elastomer lip member 102 on its rear side (a side opposite to an inner space of machine S1 of a sealing object), a lip member 104 made of a low frictional synthetic resin such as PTFE arranged on the rear side of the pressure resisting back-up ring 103, and an outer metal back-up ring 105 closely provided on the rear side of a synthetic resin lip member 104 support their each outer diameter portion in a closely adhered state each other on an inner circumference of a metal tubular case 101 which is inserted and fixed in an inner circumference of a shaft hole housing 1 through an O-ring 106.
The elastomer lip member 102 has helical grooves 102b which performs a pumping action to introduce fluid in the inner space of machine S1 on an inner circumference surface of a seal lip portion 102a of an inner diameter side extending to the inner space of machine S1 of the sealing object, thereby, a sliding load with a rotating shaft 2 is reduced, accordingly, a leakage of fluid in the inner space of machine S1 is prevented mainly when the shaft stops. Further, the elastomer lip member 102 has a support shape by the pressure resisting back-up ring 103 is set to diminish a contact width W of the seal lip portion 102a to the rotating shaft 2 at the minimum when a fluid pressure in the inner space of machine S1 is 1 MPa [gage], thereby, intends to reduce the sliding load.
Further, the synthetic resin lip member 104, mainly when the shaft rotates, prevents the sealing object fluid in the inner space of machine S1 that has passed a sliding portion of the elastomer lip member 102 from leaking toward an atmosphere S2 side.
However, if the lip seal 100 of said conventional structure is slid in high pressure condition, for example, at least 10 MPa [gage], as shown in FIG. 6, a wear C which is an unusually large cut away shape is generated on a contact surface with the rotating shaft 2 and a portion adjacent to the atmosphere side in the seal lip portion 102a, and for extreme example, the cut away wear C penetrates said seal lip portion 102a in the direction of thickness, thus a sealing function cannot be served when the shaft stops, though a lubrication by pumping action of the helical grooves 102b and limitation of the contact width W is intended for the elastomer lip member 102.
This, as a result of inventor""s study, as shown in FIG. 7, is considered because a maximum portion PMAX of a surface pressure P of the seal lip portion 102a to the outer circumferential surface of the rotating shaft 2 is locally distributed in the inner space of machine S1 so that the lubrication by pumping action of the helical grooves 102b is disturbed in the high pressure condition.
The object of the present invention is to provide a lip seal which, even if high pressure is formed in a sealing space when a shaft rotates, a lubrication of the seal lip portion of the elastomer lip member is maintained and a cut away wear is restrained, so that a sealing capability is not damaged when the shaft stops.
A lip seal according to the present invention comprises an elastomer lip member having a seal lip portion at an inner side extending toward a sealed space, which is brought into slide contact with an outer circumferential surface of a rotating shaft sealably, a rear thereof being supported with a pressure resisting back-up ring; and a synthetic resin lip member having a seal lip portion of an inner side, which is arranged at a rear side of the elastomer lip member and extends to a sealing space so as to be brought into slide contact with an outer circumferential surface of said rotating shaft at a side opposite to the sealed space from the seal lip portion of said elastomer lip member. The pressure resisting back-up ring is placed between the elastomer lip member and the elastomer lip member. Pumping threads generating a pumping action of a fluid introducing direction from said sealed space side when the shaft rotates are formed on the inner circumferential surface of the seal lip portion of said elastomer lip member. And, when a pressure of said sealing space is 1 MPa [gage], a contact width between the seal lip portion of said elastomer lip member and the rotating shaft is set to be at least 0.6 mm, so that a maximum portion of surface pressure of the seal lip portion of the elastomer lip member to the rotating shaft is locally distributed or biased toward its rear side, whereby the fluid introduction by said pumping threads can be performed satisfactorily.